LIFE SCIENCES Soy foods may reduce sperm count

Scientists are urging caution to all men who love their soy food. A research study recently published in Europe's leading reproductive medicine journal, Human Reproduction, has revealed that men who eat an average of half a serving of soy food a day have lower concentrations of sperm than their counterparts who do not eat soy foods.

For centuries, soy foods made from the soybean plant have been a staple in the diet in much of Asia. Soy is used both as a condiment as well as served as a main meal. High in protein it is also gaining popularity in the rest of the world. The results however of one of the largest studies ever conducted of soy consumption in men is that while it may be a healthy option in some respects, it may also be associated with lower sperm counts.

Dr Jorge Chavarro is a research fellow in the department of nutrition at Harvard School of Public Health, Boston, USA. Together with his colleagues they examined the relationship between semen quality and phytoestrogens. Phytoestrogens, as the name suggests, are naturally found compounds in plants that can behave like oestrogen. Oestrogen is common to both males and females, but levels are higher in women and promote female development.

The study investigated 99 men who attended a fertility clinic between 2000 and 2006. These men were then divided into four groups according to their intake of soy foods. They found that men whose dietary intake of soy was considerably higher than others had 41 million sperm per millilitre less than men who did not consume soy products. Considering that the average sperm count for men ranges from 80-120 million/ml, this reduction is close to half the count for some men.

These results were published in the medical journal Human Reproduction, which is produced by the European Society of Human Reproduction and Embryology (ESHRE) whose central aim is to promote interest in, and understanding of, reproductive biology and medicine. It also works to inform politicians and policy makers throughout Europe.

According to Dr Chavarro, 'Men in the highest intake group had a mean soy food intake of half a serving per day: in terms of their isoflavone content that is comparable to having one cup of soy milk or one serving of tofu, tempeh or soy burgers every other day,' he said.

Their results took into account varying factors such as age, abstinence time, body mass index (BMI), alcohol and caffeine intake and smoking. The group of men was also asked how often and how much they had eaten in the previous three months; the foods included tofu, tempeh, tofu or soy sausages, bacon, burgers and mince, soy milk, cheese, yoghurt and ice cream, and other soy products such as roasted nuts, drinks, powders and energy bars.

These foods were also rated against the level of isoflavones they contained. Previous animal studies have linked the high consumption of isoflavones with infertility in animals. Until now however, there has been little evidence of their effect in humans. As a result of this rating it was easier to compare intake of soy products, for example a standard serving of tofu was 115g and for soy milk it was one cup.

Based on this rating system it was easy to determine what constituted high intake, for instance. 'It is important to highlight that the figure of half a serving a day is the average intake for men in the highest intake group. Some men in this group had intakes of soy foods as high as nearly four servings per day,' said Dr Chavarro.

What was also noted by the study was the high correlation between high soy food intake, sperm levels and obesity. Dr Chavarro has speculated that the reduced production of sperm in obese men could be a result of soy foods enhancing the increased production of oestrogen, which already occurs when high levels of body fat exist.

The researchers are quick to point out that at the moment, the clinical significance of their research remains to be determined. As a result, further randomised trials are needed.